Insects and other pests cost farmers billions of dollars annually in crop losses and in the expense of keeping these pests under control. The losses caused by pests in agricultural production environments include decrease in crop yield, reduced crop quality, and increased harvesting costs.
Chemical pesticides have provided an effective method of pest control; however, the public has become concerned about the amount of residual chemicals which might be found in food, ground water, and the environment. Stringent new restrictions on the use of pesticides and the elimination of some effective pesticides from the market place could limit economical and effective options for controlling costly pests.
There is a great need for novel pest control methods which reduce the amount of pesticides necessary to obtain acceptable levels of control. Researchers have experimented with various combinations of chemicals as one approach to identify compositions which have desirable pesticidal characteristics. In the rare instance, unexpected activity of the combination of chemicals is obtained.
One group of chemicals which has been identified as having pesticidal activity is the avermectins. The avermectins are disaccharide derivatives of pentacyclic, 16-membered lactones. They can be divided into four major compounds: A.sub.1a, A.sub.2a, B.sub.1a, and B.sub.2a ; and four minor compounds: A.sub.1b, A.sub.2b, B.sub.1b, and B.sub.2b. The a and b series are sec-butyl and isopropyl homologues, respectively, which generally have similar biological activity. Despite the structural similarities to some antibiotic materials, the avermectins are believed to be devoid of antibacterial or antifungal properties.
The organism which produces avermectins was isolated and identified as Streptomyces avermitilis MA-4680 (NRRL-8165). Characteristics of the avermectin producing culture and the fermentation process are well documented and known to those skilled in the art (Burg, R. W. et al. [1979]"Avermectins, New Family of Potent Anthelmintic Agents: Producing Organism and Fermentation," Antimicrob. Agents Chemother. 15(3):361-367). The isolation and purification of these compounds is also described in U.S. Pat. No. 4,310,519, issued Jan. 12, 1982.
Another family of compounds produced by fermentation are the milbemycins, which are closely related to the avermectins. The milbemycins can be produced by a variety of Streptomyces and originally differed from the avermectins only in the C-13 position. The milbemycins and their many derivatives are also well known to those skilled in the art and are the subject of U.S. patents.
Although the avermectins were initially investigated for their anthelmintic activities, they were later found to have other insecticidal properties. There seem to be no clear boundaries of activity for the avermectins. That is, most of the compounds exhibit anthelmintic as well as insecticidal properties, although the degree varies. The activity of avermectins must generally be determined empirically.
The use of avermectins in various agricultural applications has been described in publications and patents. Specifically, the use of avermectin with spray oils (lightweight oil compositions) has been described. See, for example, U.S. Pat. No. 4,560,677 issued Dec. 24, 1985; EPO applications 0 094 779 and 0 125 155; and Anderson, T. E., J. R. Babu, R. A. Dybas, H. Mehta (1986) J. Econ. Entomol. 79:197-201.
The avermectins are reported to act by blocking neuromuscular transmission. This blockage results in immobilization of the parasite or insect. Several groups of investigators have indicated .gamma.-amino-butyric acid (GABA) release as the target system. GABA is an inhibitory neurotransmitter in vertebrates as well as invertebrates. It has been hypothesized that the avermectins cause a specific prolonged release of GABA.
22,23-dihydroavermectin B.sub.1 is a synthetic derivative of the avermectins and has been assigned the nonproprietary name of Ivermectin. It is a mixture of 80% 22,23-dihydroavermectin B.sub.1a and 20% 22,23-dihydroavermectin B.sub.1b. Ivermectin has been tested on a variety of laboratory and domestic animals for control of nematodes, ticks, and heartworms. It is applied both orally and subcutaneously.
Avermectin B.sub.2a is active against the rootknot nematode, Meloidogyne incognita. It is reported to be 10-30 times as potent as commercial contact nematicides when incorporated into soil at 0.16-0.25 kg/ha (Boyce Thompson Institute for Plant Research 58th Annual Report [1981]; Putter, I. et al. [1981]"Avermectins: Novel Insecticides, Acaracides, and Nematicides from a Soil Microorganism," Experientia 37:963-964). Residual activity was noted for up to 2 months in greenhouse tests using sandy-loam soil. The residual activity is attributed to a nematicidally active metabolite derived from avermectin B.sub.2a. The soil half-life of the metabolite is approximately one month. Nematicidal efficacy is dependent on soil-type and is least effective in organic soils. Avermectin B.sub.2a was not toxic to tomatoes or cucumbers at rates of up to 10 kg/ha.
Avermectin B.sub.1 is a combination of avermectin B.sub.1a (major component) and avermectin B.sub.1b. It has demonstrated a broad spectrum of insecticidal activities. Avermectin B.sub.1 displays a slow toxic action to insects as compared to organophosphate or pyrethroid insecticides. Insects become moribund soon after contact and die 3-4 days later. In several species a paralysis is induced which limits mobility and feeding. Once applied and dried on foliage, B.sub.1 is not rapidly degraded by sunlight nor washed away by rain.
The data indicate that avermectin B.sub.1 is primarily a miticide, although it is also effective on the Colorado potato beetle, potato tuberworm, beet armyworm, diamondback moth, gypsy moth, and the European corn borer. Marginal activity has been found on several other species.
Use of avermectin B.sub.1a on the imported fire ant (Solenopsis invicta) has been found to permanently halt egg production in queen ants (Putter et al., supra). Death of worker ants seems to be a secondary effect which occurs more frequently at high dose rates. The mechanism by which this happens is not known.
Among the pests against which the novel compositions of the instant invention are active are mites, leafminers, whiteflies, psylla, and fire ants. These are important pests, as described below.
Feeding damage caused by the two-spotted spider mite (TSM), Tetranychus urticae is initially manifested as a stippling of the plant leaves and chlorophyll damage. Subsequent symptoms include a yellowing or silvering of the leaves followed by eventual defoliation. Hatching eggs pass through one larval and two nymphal stages reaching adulthood in 8-12 days. Each female may lay up to 100 eggs in her 30 day lifespan. TSM occur on the underside of leaves, making coverage difficult. Overlapping generations confer a great biotic potential to TSM, resulting in outbreaks and contributing to the development of insecticide resistance. TSM are ubiquitous in that they are found in greenhouses and on vegetables, ornamentals, and fruit.
Female leafminers, Liriomyza trifoli, become active at dawn, feeding on leaf juices by repeatedly puncturing the leaf surface. Males feed from the punctures made by the female. Approximately 15 % of these feeding holes are used for egg-laying. Each female can lay approximately 250 eggs in her 30 day life span. Eggs hatch in 3-5 days, with the resulting maggots causing the characteristic "mines" as they tunnel and feed throughout the leaf, reducing photosynthetic rates and leading to reduced plant growth and development. Under favorable conditions, the life cycle can be completed in three weeks or less. If left unchecked, several overlapping generations can mature to further infest the crop. The development of insecticide resistance and reduction of the associated beneficial insect complex brought on by widespread use of broad spectrum insecticides has elevated leafminers from a minor to major pest status in ornamentals and vegetables such as chrysanthemums, carnations, tomatoes, and celery. California celery growers lost approximately $20 million in the last half of 1984 due to L. trifoli.
Whiteflies, mites, aphids, thrips, mealybugs, and other pests cause millions of dollars of damage each year to ornamental plants and plants grown in greenhouses. For example, the sweetpotato whitefly, Bemisia tabaci, is widely distributed throughout tropical and subtropical areas north and south of the equator. B. tabaci is a primary pest of cotton, ornamentals, and vegetables, both in the field and in the greenhouse. During 1981, B. tabaci was responsible for crop and market losses of $100 million in cotton, cucurbits, and lettuce and California and Arizona. The whitefly is increasingly a problem in Florida where, in 1986, B. tabaci caused approximately $2 million of damage to Florida's $8-10 million poinsettia crop. This insect is now known to feed on more than 500 different plants, many of which are of importance in the Caribbean and Florida. For example, cassava, sweet potato, squash, tomato, beans, lettuce, cotton, pepper, carrot, cucumber, eggplant, and watermelon are all known hosts. Sweetpotato whitefly advance from the newly laid egg, through the crawler and scale-like nymphal stages, through the pupal stage, to the newly emerged adult whitefly in approximately 3-4 weeks. Whiteflies may cause direct damage at very high densities through sap removal. Indirect damage may occur through virus and mycoplasma plant disease transmission. Production of honeydew may result in sooty mold contamination. Variability in time required for development among individuals within a population, and frequent arrivals of adults from the neighboring environment, ensures populations of mixed life stages. Treating all life stages of a pest, as opposed to discrete life stages, increases the rate of resistance development, with whiteflies being no exception. B. tabaci (Gennadius) has proven to be very difficult to control with conventional pesticide applications. Many factors contribute to the lack of control obtained with pesticides. The most important factor is that this white fly has demonstrated a broad spectrum of resistance to chlorinated hydrocarbon, organophosphorus, carbamate, and synthetic pyrethroid insecticides. Very few commercially available pesticides are effective against whiteflies, and those which do work are only effective if care is taken to make a very thorough application of the insecticide several times a week.
The pear psylla, Psylla pyricola, is a primary pest in all major pear-growing areas of North America, causing leaf abscission and a reduction in fruit size, root growth, tree growth, and fruit set in years following high psylla populations. Pear psylla are characterized by a distinct summer and overwintering adult form. There may be from two generations in Ontario to as many as five in California. Applications of insecticide are currently timed to coincide with early season egg deposition or tree developmental stages such as delayed dormant, petal fall, midsummer, and post-harvest. Fruit russet from honeydew contact is the principal injury caused by pear psylla, becoming evident approximately seven days after honeydew contact with the fruit. A sooty mold usually develops in the honeydew and blackens the affected tissue. Feeding by large populations of psylla can result in "psylla shock" characterized by reduced vigor, fruit loss, and poor fruit set. Toxins in the salivary secretions interfere with food transfer in susceptible cultivars, causing "pear decline" resulting in slow or rapid death of the tree. Natural enemies alone do not exert a significant influence on psylla populations, however, a "soft" insecticidal regime in combination with these natural enemies should provide economically acceptable levels of control.
The imported fire ants (IFA), Solenopsis invicta and Solenopsis richteri were introduced into the United States in the early 1900's at the port of Mobile, Alabama. Imported fire ants displace the native ant fauna and, in many areas of the southeastern United States, have become the dominant ant species. Currently, the imported fire ant has gained notoriety primarily as a result of its painful sting and its inclination to feed on a variety of materials, including cultivated plants and underground wires. The fire ant sting is not only painful but is potentially life threatening for people who suffer from an allergic reaction to the sting. Farmers throughout the southern states have suffered large economic losses as the result of fire ant infestations. Fire ants reduce the active foraging area in pastures because animals do not forage well around fire ant nests. Fire ants may also damage plants by chewing on stems or fruits. IFA has been reported to cause serious damage in young citrus groves and in vegetable crops with high cash values. Also, these ants make it difficult to harvest some crops such as hay and citrus. Large fire ant mounds may also cause damage to agricultural equipment, especially in heavy clay soil areas. Additional economic loss has resulted from the IFA chewing on electrical wiring and telephone lines in the ground or even housed in containers above the ground.